A variety of systems are available that employ nucleic acid vector molecules for the expression of recombinant proteins in a diverse and wide variety of eukaryotic or prokaryotic host cells. The decision of which vector/host cell pair to use typically depends upon which system provides the highest yield of the desired recombinant gene product in a form best suited for its intended use. For example, if a post-translational modification, such as glycosylation, is critical to requirements of the desired recombinant protein (e.g., for antigenicity, activity, conformation, etc.), then a eukaryotic host cell system will be desired as prokaryotic cells are characteristically devoid of post-translational glycosylation activity. It is also important that a vector/host cell pair not only produce an expressed gene product in the desired form, but also that molecules of the desired expressed gene product can be readily isolated from the cells that produce them.
Owing to the escalating costs involved in the development of recombinant proteins intended for human therapy, there remains an ongoing effort to enhance the level of expression of such recombinant proteins, especially in systems that employ eukaryotic host cells. Various cis- and trans-acting regulatory elements have been characterized that have nucleotide sequences (DNA or RNA) that may improve efficiency of expression and/or overall yield of the desired recombinant gene product. Such regulatory elements include, but are not limited to, highly efficient promoters, transcriptional enhancer sequences (see, e.g., the review by Dillon and Grosveld, Trends Genet., 9: 134 (1993), locus control regions (LCRs; see, e.g., Grosveld et al., Cell, 51: 975 (1987)), matrix or scaffold attachment regions (MARs, SARs; see, e.g., U.S. Pat. No. 7,129,062; Bode et al., Int. Rev. Cytol., 162A: 389-454 (1995); Bode et al., Crit. Rev. Eukaryotic Gene Expression, 6: 115-138 (1996)); insulator elements (see, e.g., Kellum et al., Cell, 64: 941 (1991); and internal ribosome entry sites (IRES; see, e.g., review by McBratney et al., Curr. Opin. Cell Biol., 5: 961 (1993)). Some of the nucleic acid sequences of such elements have been subsequently found to possess specific subsequences that can affect expression.
Despite advances in the understanding of various sequences and other factors that can affect expression of recombinant proteins in various types of host cells, needs remain to improve the yields of recombinant proteins, particularly when such recombinant proteins are intended for therapeutic or other specialized applications.